High reflectivity, high heat radiating properties, dielectric strength voltage, and long-term reliability can be exemplified as basic properties necessary for a substrate for light emitting devices. High dielectric strength voltage properties are particularly necessary for a substrate for light emitting devices used in high-intensity lighting.
In the related art, a light emitting device including a ceramic substrate or a substrate including an organic resist layer on a metal base as an insulating layer has been known as a substrate for light emitting devices. Hereinafter, respective problems of a ceramic substrate and a substrate using a metal base will be described.
(Ceramic Substrate)
For example, a ceramic substrate is manufactured by forming an electrode pattern on a plate-like ceramic base. The brightness is required to be improved by arranging a plurality of light emitting elements on the substrate, in accordance with a tendency of realizing high output of a light emitting device, and therefore, ceramic substrates have been increasing in size every year.
Specifically, in a case of realizing a typical LED (light emitting diode) light emitting device using supplied power of 30 W by arranging, for example, face-up type (an active layer is positioned to be far from a mounting surface) blue LED elements having a dimension of 650 μm×650 μm or approximately the dimension thereof in one substrate classified as a medium-size substrate, approximately 100 blue LED elements are necessary. As a ceramic substrate in which approximately 100 blue LED elements are arranged, a ceramic substrate having a horizontal size of 20 mm×20 mm or larger and a thickness of approximately 1 mm is used, for example.
In a case where it is attempted to realize a light emitting device for brighter LED lighting using supplied power of 100 W or higher, a large-sized ceramic substrate having at least a horizontal size of 40 mm×40 mm or larger which can mount 400 or more blue LED elements at once is necessary, as a result of technological development based on an increase in size of a substrate.
However, increasing a size of a ceramic substrate has been attempted in order to realize a larger size on a commercial basis in accordance with the requirements of an increase in size of a ceramic substrate, but a larger size thereof is difficult to realize on a commercial basis due to three problems of strength, manufacturing precision, and manufacturing cost of a ceramic substrate.
Specifically, since a ceramic material is normally a piece of pottery, a problem regarding strength of a ceramic substrate occurs, when a size thereof is increased. When a thickness of a substrate is increased in order to solve this problem, new problems occur such that heat resistance increases (heat radiating properties are deteriorated) and a material cost of a ceramic substrate also increases. When a size of a ceramic substrate is increased, not only external dimensions of a ceramic substrate, but dimensions of an electrode pattern formed on a ceramic substrate are also likely changed, and this may easily result in a decrease in manufacturing yield of a ceramic substrate and an increase in manufacturing cost of a ceramic substrate.
An increase in the number of light emitting elements to be mounted on a ceramic substrate is also a problem along with the problems associated with an increase in size of a ceramic substrate. For example, in the light emitting device described above, the number of light emitting elements mounted on one ceramic substrate is extremely large, as much as 400 or more, and this is a reason for a decrease in manufacturing yield.
In addition, in a face-up type light emitting element, an active layer is positioned on a side far from a light emitting element mounting surface of a substrate for light emitting devices, and accordingly, heat resistance up to a portion of an active layer is high and an active layer temperature easily increases. In a high-output light emitting device having the large number of light emitting elements to be accumulated on one ceramic substrate, a substrate temperature which is a basic condition is high, an active layer temperature of a light emitting element is further increased, together with the substrate temperature, and a decrease in lifetime of a light emitting element becomes significant.
(Substrate Using Metal Base)
Meanwhile, a metal base having high thermal conductivity may be used as a substrate for high-output light emitting devices, in order to solve the problems in a ceramic substrate. Here, an insulating layer is provided on a metal base, in order to mount light emitting elements on a metal base and in order to form an electrode pattern connected to the light emitting elements.
In a substrate for light emitting devices, an organic resist is used as a material used as an insulating layer, in the related art.
In addition, it is necessary that the insulating layer has high light reflectivity, in order to improve light use efficiency in a substrate for high-output light emitting devices.
However, in a case of using an organic resist as an insulating layer in a substrate for light emitting devices in the related art, sufficient thermal conductivity, heat resistance, and light resistance are not obtained and dielectric strength voltage properties necessary for a substrate for high-output light emitting devices are not obtained. It is necessary that light leaked to a metal base side through an insulating layer is reflected, in order to improve light use efficiency, but sufficient light reflectivity is not obtained in a configuration in which an organic resist of the related art is used as an insulating layer.
Therefore, a substrate in which an insulator layer is formed on a substrate using a metal base, by using a ceramic coating material has been proposed.
It is possible to realize a substrate for light emitting devices having excellent reflectivity, heat resistance, and light resistance, in a substrate for light emitting devices in which a light reflecting layer which also serves as an insulator layer is formed on a metal base surface by using a ceramic coating material. PTL 1 discloses a forming method of the light reflecting layer which also serves as an insulator layer of applying a ceramic coating material onto a base.
PTL 5 below discloses a technology of forming an insulating layer formed of ceramic such as alumina on a metal base which is a base by plasma spraying, without using a coating material, and manufacturing a substrate for a light source, for example. In the substrate for a light source in which an alumina insulating layer is formed by plasma spraying as described above, it is possible to realize a substrate for a light source having excellent electrical dielectric strength voltage properties.
In addition, PTL 6 below discloses a technology of forming a ceramic layer on a surface of a metal substrate by using an aerosol deposition method (hereinafter, also referred to as an “AD method”).